F Head Compression Woes

I finally got around to doing a compression test on my F-Head. I was simply looking for 4 consistent numbers in the ball park, just to make sure my engine is not junk before I proceed with my project. If the engine is crap, then I will likely scrap the entire project.

I ran the engine at idle for maybe a minute. With ambient temps around 45F, the compression numbers might be low, but again, all I want is 4 consistent numbers. Cylinder #1 came in around 95, cylinder #2 came in at 110. I re-ran cylinder #1, and it came up to 110. Not impressive, but nothing to be concerned about. Then, Cylinder #3 shows low 30s. Ruh roh roge. Cylinder #4 is at 95. Retry #3, and again in the lower 30s. I warm up the motor, and cyl #3 increases to 40. Crap. I tested cyl #3 several more times, with very consistent results.

The cylinder #3 compression is so low that I have to think that the problem is either a valve or a head gasket.

All 4 plugs look the same - black. The engine runs pig rich for some reason. Exhaust is smoggy - was hoping that it is fuel and not oil, but not certain now.

Any suggestions?

 

"If the engine is crap, then I will likely scrap the entire project"

Only one cylinder is really bad... A head gasket or valve is likely, and not a big deal - a gasket might only be half a day's work.
But of course with the other numbers being low-ish, you might decide it's time to refresh the engine with new rings and a valve job.

Possibly upon inspection it will need a full rebuild. How is your oil pressure?

Black, evenly "sooty" dry plugs indicate excess gasoline. Wet looking, with heavy dark deposits of crud on them, is more likely a sign of oil burning.

 

Given that I probably have a worn synchro, I am just not motivated to invest the time or the money.

. Yep, evenly "sooty" dry plugs.

No idea on the oil pressure, other than the oil light is not light when running.

 

I had compression "issues" with mine, as well as running outrageously rich...
I went to the local boat shop and ran mercury marine power tune through it, and really soaked the rings... It cleared it right up! Rebuilt the carburetor because I couldn't figure out why it was running so rich, I think it was a float level issue, not sure, runs as well as possible now... (raging 72 hp from the factory! Rock On!)
I'd recommend something to see if your rings are stuck first (my plugs were wet and sticky- not sooty)... Would be a quick, simple thing..
Hope it helps.

 

How did you soak the rings - did you just pour some directly into the cylinder via the spark plug hole?

 

You actually squirt the stuff down the carb while it's running (warm engine), it'll smoke to high heaven, just run as much as you can into the engine without killing it for a couple of minutes, then just give it all she's got until engine dies. This will soak the rings. Let it sit for an hour or so, then start it up, and take it for a spin.. Again, it will smoke a LOT when you first start it up..
After you take it for a spin, you'll want to change the plugs and the oil, but I would encourage you to check the compression before that.
I gave mine four treatments before changing oil and plugs.
Hope it helps.